The present invention relates generally to power conversion systems and deals more specifically with a method for selecting pulse width modulation waveform patterns using weighted factors to control a multiple bridgepower inverter to produce a multiple phase AC voltage output signal having low harmonic signal content.
The problem of producing harmonic free AC output power from a power inverter exists for any known means of generation. It is also known that harmonics in the AC output power are generally more severe when the input to the power inverter is from a switched DC voltage source. The use of high power semiconductor inverters has fostered a number of different approaches to reduce harmonics in the AC voltage output; however, these approaches have generally not been satisfactory. The use of passive filters to reduce harmonics is limited due to losses, cost and unsatisfactory dynamic performance.
Although not completely satisfactory, two methods for reducing harmonics in inverter output voltages have been used with some degree of success. One method of harmonic reduction utilizes combinations of phase displaced, three-phase bridges and reference may be made to U.S. Pat. No. 4,975,822 assigned to the same assignee as the present invention and which disclosure is incorporated herein by reference.
A second known method to achieve harmonic reduction includes exciting the power inverter utilizing a pulse width modulated control signal. The improvement in semiconductor switching devices to handle high power levels has permitted the design of power inverters to produce several hundred kilowatts and pulse width modulation techniques have been used to remove a substantial portion of the unwanted harmonics in the power inverter bridge output voltage signal.
It is not easy to define a given pulse width modulated waveform signal which has the desired harmonic reduction because the waveforms are expressed either as a Fourier series of sine and cosine terms or Bessel functions. One commonly known method to define a pulse width modulated waveform uses the sine-triangle method to generate gating signals to control the semiconductor switching. One problem generally associated with the sine-triangle method is that the peak of the sinewave cannot reach the apex of the triangle and the modulation index over the full switching range approximates 80% whereas a high as possible modulation index is desired.
Also, the sine-triangle method generally results in an inverter output voltage signal having a very high magnitude 11th and 13th harmonic. The high harmonics often lead to high overcurrents which trip the circuit breakers associated with each output and also contributes to a "noisy" output voltage signal.
Another factor to be considered in obtaining a suitable PWM waveform pattern is that harmonics which are reduced at one frequency generally always result in increasing other harmonics at different frequencies. Additional problems generally associated with pulse width modulation techniques for reducing harmonics is that each switch pair added to the PWM waveform pattern reduces the available voltage from the inverter.
There are also several practical considerations that limit the use of the pulse width modulation PWM techniques for reduction of harmonic signals and include among them: dwell time when both switch base drives must be in an off-state to compensate for the semiconductor device turnoff delay; limitations in the minimum pulse width obtainable due to the control design and upper frequency limits of the switch devices used. Although PWM techniques have made possible some improvements in the reduction of harmonics, the reductions are not always predictable and the same harmonics are often cancelled more than once.
It is a general aim of the present invention therefore to provide a method to design a set of pulse width modulation (PWM) waveform patterns that reduce only those harmonics necessary for a particular design and which harmonics are not removed by other portions of the power inverter system. Accordingly, the method of the present invention permits the selection of PWM waveform patterns that eliminate or otherwise minimize the undesired harmonics to an acceptable magnitude.